1. Field of the Invention
The present invention relates to an absolute encoder having a scale section in which an absolute code is etched onto the surface of a magnetic material.
2. Description of the Related Art
A prior-art technique will be described with reference to FIG. 7. FIG. 7 shows an absolute encoder described in Patent Document 1 (Japanese Patent Publication JP-A-5-118874). An encoder disc 7 includes an incremental gear shape component for detecting a two-phase sinusoidal signal, and a concave/convex portion conforming to a binary cyclic random number sequence, for detecting an absolute pattern. In addition, magnetic sensors 5s and 5c, which are made of magnetic reluctance elements and which detect the two-phase sinusoidal signal, and magnetic sensors 1a-1d, 2a-2d, 3a-3d and 4a-4d, which detect the absolute pattern, are configured so as to detect the magnetic reluctance change between a permanent magnet not shown and the encoder disc 7. In general, the permanent magnet (not shown) is disposed on the opposite side to the encoder disc 7, that is, on a back side (rear side) as viewed from the magnetic sensors 5s and 5c and those 1a-1d, 2a-2d, 3a-3d and 4a-4d. Here, it is obvious that the concavities and convexities affixed to the outer peripheral part of the encoder disc 7 change depending upon the rotational position of the encoder disc 7. In this regard, at a rotational position at which the number of the convex portions is large, the total quantity of magnetic flux which passes from the permanent magnet to the encoder disc 7 through the magnetic sensors 5s and 5c and those 1a-1d, 2a-2d, 3a-3d and 4a-4d becomes large, whereas at the rotational position at which the number of the concave parts is large, the total quantity of magnetic flux which passes from the permanent magnet to the encoder disc 7 through the magnetic sensors 5s and 5c and those 1a-1d, 2a-2d, 3a-3d and 4a-4d becomes small. Especially, regarding the concave/convex portion of the encoder disc 7 conforming to the binary cyclic random number sequence, the proportion of the concave parts or the convex parts changes extremely, depending upon the rotational position, and, hence, the change of the total magnetic flux quantity from the permanent magnet is also violent. Regarding the concave/convex portion of the encoder disc 7 for detecting the two-phase sinusoidal signal, the change of the total magnetic flux quantity from the permanent magnet is slight owing to the incremental gear shape. However, in a case where the dimension of the permanent magnet in the rotating direction of a gear is not integral times the pitch of the incremental gear shape, the total magnetic flux quantity from the permanent magnet changes depending upon the rotational position within one pitch of the gear shape. Additionally, because the concave/convex portion for the binary cyclic random number sequence excerpts an influence in three dimensions, magnetic flux quantities from the permanent magnet vary as it passes through the magnetic sensors 5s and 5c depending upon the rotational position, due to factors other than the magnetic reluctance change relating to the incremental gear shape.
Next, concerning the change of the total magnetic flux quantity passing through the magnetic sensors 5s and 5c and those 1a-1d, 2a-2d, 3a-3d and 4a-4d from the permanent magnet, basically the magnetic sensors 5s and 5c and those 1a-1d, 2a-2d, 3a-3d and 4a-4d detect the gap quantity changes between them and the concave/convex portion of the encoder disc 7, as magnetic reluctance changes. Therefore, change of the magnetic flux quantity from the permanent magnet ascribable to any other factor becomes a detection error. Especially in the magnetic sensors 5s and 5c for detecting the two-phase sinusoidal signal, deterioration in detection precision has often occurred.
Next, a prior-art absolute encoder different from that of Patent Document 1 (JP-A-5-118874) will be explained. FIG. 5 is a perspective view showing an example of the detection portion of the prior-art absolute encoder. A rotary shaft 501 is supported in a plurality of bearings (not shown), so as to be rotatable. A gear 502 for detecting a two-phase sinusoidal signal and a binary cyclic random number sequence code plate 503 are fixed to the rotary shaft 501. Magnetic sensors 504a-504d for detecting the two-phase sinusoidal signal, which are made of magnetic reluctance elements, are disposed so as to oppose to the outer peripheral surface of the gear 502 for detecting the two-phase sinusoidal signal and to be spaced therefrom by predetermined gaps. Magnetic sensors 504e-504n for detecting a binary cyclic random number sequence, which are made of magnetic reluctance elements, are also disposed  so as to oppose to the outer peripheral surface of the binary cyclic random number sequence code plate 503, and to be spaced therefrom by predetermined gaps. A core 505a for exciting the two-phase sinusoidal signal, which is made of a magnetic material such as ferrite, is disposed near the back surfaces of the magnetic sensors 504a-504d for detecting the two-phase sinusoidal signal. Similarly, a core 505b for exciting the binary cyclic random number sequence is disposed near the back surfaces of the magnetic sensors 504e-504n for detecting the binary cyclic random number sequence.
An excitation method for the two exciting cores 505a and 505b will be explained with reference to FIG. 6. FIG. 6 is a sectional view showing excitation magnetic paths in an example wherein an excitation winding is wound on an E-shaped exciting core. The winding 606 is wound on the middle convex part of the E-shaped exciting core 605, and the magnetic paths indicated by arrows are formed outward of the E-shaped exciting core 605. The E-shaped exciting cores 505a and 505b are separated for the two-phase sinusoidal signal and for the binary cyclic random number sequence, whereby the signal and the sequence are endowed with respective dedicated excitation magnetic paths. Because when such excitation magnetic paths are formed magnetic flux which forms a loop is generated by the E-shaped exciting core itself, the configuration is advantageous in that the amount of magnetic flux lost is smaller than with excitation magnetic paths based on the shape of a bar magnet.
However, the magnetic paths formed by the E-shaped exciting cores 505a and 505b are subject to three-dimensional interferences. More specifically, there appear loops in which the magnetic flux generated from the middle convex part of the exciting core 505a arrive at the right and left convex parts of the exciting core 505b, and loops in which the magnetic flux generated from the middle convex part of the exciting core 505b arrive at the right and left convex parts of the exciting core 505a. The influence of the interferences of the excitation magnetic flux, and the influence under which the total magnetic flux quantity of the excitation magnetic flux from the exciting core 505b changes depending upon the proportion of the convex parts or concave parts of the concave/convex portion of the binary cyclic random number sequence code plate 503, act together to change the total magnetic flux quantity of the excitation magnetic flux passing through the magnetic sensors 504a-504d for detecting the two-phase sinusoidal signal, and to form the factor for hampering the detection precision. Additionally, the E-shaped exciting cores 505a and 505b become larger than the magnetic sensors 504a-504n which are arranged in the tangential direction of the gear. Especially, among the magnetic sensors 504a-504n, the magnetic sensors 504e-504n for the binary cyclic random number sequence are arranged to be longer in the tangential direction of the gear, so that the E-shaped exciting core 505b has become larger. This has led to the disadvantage that the size of cases for accommodating a sensor section (not shown) had to be increased.
In the absolute encoder stated in Patent Document 1 and illustrated in FIG. 7, the total magnetic flux quantity generated from the permanent magnet changes depending upon the proportion of the convex parts or concave parts of the concave/convex portion of the encoder disc 7, so that a detection error might occur in the detection of the binary cyclic random number sequence by the magnetic sensors 1a-1d, 2a-2d, 3a-3d and 4a-4d. Also in the detection of the two-phase sinusoidal signal by the magnetic sensors 5s and 5c, because the total magnetic flux quantity of excitation magnetic flux varies due to the influence of the proportion of the convex parts or concave parts of the concave/convex portion of the binary cyclic random number sequence, precision has been impaired. Even in the example of FIG. 5, in which the excitation means for detecting the two-phase sinusoidal signal and the excitation means for detecting the binary random number sequence are separated in order to suppress this influence, the excitation magnetic paths generated by the E-shaped exciting cores interfere in three dimensions, so that a precision has been spoilt in the detection of the two-phase sinusoidal signal by the magnetic sensors 504a-504d. Meanwhile, from the viewpoint of size, when the E-shaped exciting cores are adopted, a dimension longer than a dimension in that tangential direction of the gear along which the magnetic sensors 504a-504d and 504e-504n are arranged is required of the exciting cores 505a and 505b, and the size of sensors increased. On the other hand, because the permanent magnet and the winding are used for the excitation means, an excitation magnetic flux quantity is liable to change due to the temperature characteristic of the permanent magnet or the temperature change of a winding resistance. Accordingly, there has been a problem that, when a temperature has changed, the output levels of the magnetic sensors 504e-504n for detecting the binary cyclic random number sequence code has fluctuated, increasing the likelihood of error in detection of the binary cyclic random number sequence code.